1. Field of the Invention
The present invention relates to a communication device and a method used in a wireless communication system, and more particularly, to a communication device and method of handling data transmission in an unlicensed band in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3GPP as a successor of the universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes at least one evolved Node-B (eNB) for communicating with at least one user equipment (UE), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, increases peak data rate and throughput, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmissions/reception, uplink (UL) multiple-input multiple-output (UL-MIMO), licensed-assisted access (LAA) using LTE, etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Network operators propose to offload network traffic of the LTE/LTE-A system to an unlicensed band, to ease load of the network traffic. For example, the eNB may provide (e.g., allocate, assign) services to the UE via the unlicensed band. However, resource (e.g., service) provided by the eNB may not be available due to listen before talk (LBT) in the unlicensed band. In this situation, the UE is not able to perform a transmission (e.g., data transmission) via the resource. Correspondingly, the eNB is unaware of whether the transmission is performed. An error may occur, if the UE does not perform the transmission due to the LBT and the eNB proceeds to perform a hybrid automatic repeat request (HARQ) process corresponding to the transmission. That is, the eNB treats the transmission as being incorrect rather than not being performed. As a result, not only the performance of the HARQ process is degraded, but the benefit of the usage of the unlicensed band is also diminished.
Thus, how to handle the transmission in the unlicensed band is an important problem to be solved.